Polyamide resins are widely, used in various application fields because they have excellent mechanical properties and can be improved remarkably in their strength and rigidity by reinforcement with glass fiber. However, polyamide compositions containing glass fiber at a relatively higher content for the improvement of the strength and rigidity have problems, for example, such that articles molded by injection molding have poor appearance owing to emergence of the glass fiber on the molded article surface.
To solve such problems, a composition is disclosed which employs a polyamide having a relatively low crystallization temperature among polyamide resins such as those derived by copolymerization of adipic acid and m-xylylenediamine (hereinafter referred to as "nylon MXD6") or by copolymerization of adipic acid, isophthalic acid and hexamethylenediamine (hereinafter referred to as "nylon 66/6I copolymer"), and which polyamide is reinforced by glass fiber.
As other methods for improving the appearance of the molded articles, for example, JP-A-56-30460, and JP-A-63-156856 (The term "JP-A" as used herein means an "unexamined published Japanese patent application.) disclose resin compositions composed of a polyester resin or a nylon 46 resin and glass fiber incorporated therein, in which the glass fiber length is regulated for improving the balance between external appearance and mechanical properties. The aforementioned fiber-reinforced resin compositions produce molded articles having good appearance by preventing emergence of glass fiber out of the article surface to a certain extent. However, such compositions are not yet satisfactory for uses requiring more excellent appearance such as surface smoothness free from periodic roughness of about 1 to 5 mm period generated on a molded article surface (hereinafter referred to as "waviness"), and do not give sufficient mechanical properties of the resulting molded articles. In particular, nylon 46 has a crystallization temperature as high as 250.degree. C. and does not produce a molded article having satisfactory appearance. Moreover, nylon 46 cannot be readily molded owing to the narrow range of the applicable molding conditions.
To solve the above problems, the inventors of the present invention paid attention to the glass fiber in the glass fiber-reinforced polyamide resin. They found that the polyamide resin composition is improved in the surface smoothness and the balance of mechanical properties by regulating the average aspect ratio of the glass fiber and the content of a fraction having a high aspect ratio in the glass fiber. Thus, an application has been filed for the invention as disclosed in JP-A-10-120900.
The aforementioned polyamide composition, however, is not satisfactory yet for use in application fields which require higher mechanical strength, especially higher level of impact strength, more excellent appearance (gloss and surface smoothness), and higher weatherability (especially, resistance to phenomena that articles under conditions where they are used are deteriorated by exposure to ultraviolet light and rain (water) to cause emergence of the glass fiber on the article surface, thereby whitening the article with loss of gloss).